The present invention relates to an instrumented data acquisition system for monitoring and recording operational characteristics of heavy or medium duty vehicle power steering systems and a method of using same in testing the operation and response of a steering system in a heavy or medium duty truck including school buses. More specifically, the apparatus allows a technician to measure all steering system parameters necessary to test functionality. The instrumented data acquisition system is the aggregate of sensing devices which may be installed in its entirety or specific parts to monitor steering system performance. It is expected the instrumented data acquisition system with all its parts will be a diagnostic tool which will be removed from the vehicle at the completion of testing and trouble shooting. One sub-component of the instrumented data acquisition tool that may be permanently installed on a vehicle is the steering wheel rotation measuring assembly. The steering wheel rotation measuring assembly may be installed to obtain an accurate signal directly representing the amount of steering wheel turning by an operator. This signal could either be recorded on paper or in a computer hard drive, displayed in real time on the dashboard of the vehicle, or used to provide a warning alarm to a vehicle driver.